Question

I have a classes defined as so:

public abstract class AbstractUploadField<T> {

And

public class GroupField<T> extends AbstractUploadField<T> {

Then in Java I am trying to use generics to make sure a class is a subclass of AbstractUploadField

Class<? extends AbstractUploadField<?>> clazz = GroupField.class

The above does not compile, but if I remove the second wildcard it does:

Class<? extends AbstractUploadField> clazz = GroupField.class

I don’t understand why the first assignment wouldn’t work

Answer 1

GroupField and AbstractUploadField should both have the same generic type T

In the line

Class<? extends AbstractUploadField<?>> clazz = GroupField.class

The first wildcard can be different from the second wildcard which contradicts the first statement.

The statement:

Class<? extends AbstractUploadField> clazz = GroupField.class

works but I assusme you are getting a warning for it.